In a system (2) having a balloon catheter (1) with a balloon (7) and an elongated catheter (3), a planimetry measuring system (19) in the balloon, and a linear distance measuring element (23) slideable along the elongated catheter from a proximal end (4) thereof to the datum location (8) when the balloon catheter has been inserted through the arterial system with the balloon located in the valve orifice (17), a plurality of secondary optically detectable elements (30) are equi-spaced longitudinally along the catheter and an optical encoder (32) in the linear distance measuring element counts the detectable elements as the linear distance measuring element is moved from a reset position to the datum location. A signal processor (20) reads signals from the linear distance measuring element and from the planimetry measuring system for determining the distance of the valve orifice of the aortic valve (9) from the datum location.
